Composites are important structural materials. Oftentimes composites are reinforced by suspending or embedding solid strengthening or reinforcing elements, such as, reinforcing powders, rods, sheets, weaves, or combinations thereof within the composite matrix. Generally, the solid reinforcing elements are rigid and temperature resistant and are thus used to make the entire composite matrix more rigid and temperature resistant. Many other benefits are achieved by reinforcing composites. For example, reinforced composites can be prepared which resist creep, fatigue, and tensile or shear fractures at temperatures which are close to the melting point of the composite matrix.
Reinforced composites are formed by adding solid strengthening or reinforcing elements to a liquid composite matrix followed by mixing, in an attempt to achieve a homogeneous mixture, and finally solidifying or freezing the mixture to provide a reinforced composite matrix which contains the solid strengthening or reinforcing elements embedded therein. Ideally, the solid reinforcing elements are uniformly distributed in the composite matrix to realize and optimize the desired performance of the reinforced composite matrix. However, it is extremely difficult, if not impossible, to achieve uniform distribution of reinforcing elements in a composite matrix.
The uniform distribution of the solid reinforcing elements in a liquid or solid composite matrix is a critical factor in achieving optimum composite performance. If the solid reinforcing elements are heavier than the composite matrix, they gravitationally segregate at the bottom of the liquid composite matrix during the solidification process. This segregation causes a non-uniform distribution of reinforcing elements in the composite matrix. Specifically, the solid reinforcing elements are overcrowded at the bottom of the composite matrix if the reinforcing elements are heavier, or have higher densities, than the composite matrix. This overcrowding reduces the efficacy of these solid reinforcing elements and decreases the usefulness of the resulting reinforced composite.
Reinforcing element segregation at corners, edges, and deep but narrow walls is also very common. Overcrowded reinforcing elements at certain segregated places, such as the bottom for heavier solid reinforcing elements or the top for lighter solid reinforcing elements, causes weakness in the composite matrix. In particular, if a composite matrix has too many solid reinforcing elements, it may be even weaker than a composite matrix without any reinforcement. This weakness results because the solid reinforcing elements are not sufficiently supported by, or connected to, the composite matrix which causes localized overstresses, which in turn initiate voids and cracks in the matrix. Similarly, in areas of the composite matrix where solid reinforcing elements are underpopulated, the composite matrix is, of course, weak and not properly reinforced.
Proper reinforcement is also problematic in cases where a composite is narrow and deep, such as between two concentric cylinders. In this case the composite thickness between the inner and outer cylindrical walls may be one inch to several inches long and the radius or thickness of the solid reinforcing elements may be 10 microns to 3 mils. Given these parameters, the gravitational segregation of solid reinforcing elements at localized spots may initiate premature composite failure.
An inferior composite results because of the differing densities of the liquid composite matrix and the solid reinforcing elements. In particular, solid reinforcing elements sink when suspended in a lighter liquid composite matrix. On the other hand, lighter reinforcing elements float when suspended in a heavier liquid composite matrix. In either case, the solid reinforcing elements segregate due to gravity, resulting in a non-uniform distribution of the solid reinforcing elements in the liquid composite matrix. Further, this non-uniform distribution pattern is carried over during the composite matrix solidification, e.g., freezing or resin polymerization of the composite matrix, resulting in undesirable segregation patterns of the solid reinforcing elements in the resultant solid composite matrix.
This non-uniform distribution of reinforcing elements which is detrimental to the performance of a reinforced composite is shown in FIG. 1, which is a cross-sectional view of a prior-art reinforced composite, taken along lines 1--1 thereof, and is denoted as a whole by reference numeral 10. As shown in FIG. 1, the prior art reinforcing elements 11 have a non-uniform distribution in the composite matrix 12.
Different approaches, having varying degrees of success have attempted to overcome the deficiencies in the prior art reinforced composites. Specifically, a tedious and time-consuming process of hand packing reinforcing elements into a composite matrix has attempted to achieve the desired reinforcing element uniform distribution. In particular, alternate sheets of composite matrix of a first thickness and solid reinforcing sheets or two-dimensional weaves of a second thickness may, for example, be hand-packed together, layer after layer, followed by liquid infiltration and freezing, pressing or thermal polymerization to form a resultant reinforced composite matrix. This process has several shortcomings, including non-uniform distribution of the reinforcing elements caused by shifting or settling of the packed material, irreproducibility of results and excessive expense in forming the reinforced composite.
Another approach which has attempted to provide uniform distribution of reinforcing elements uses a process which suspends the solid reinforcing elements in a liquid or molten composite matrix. This suspension is then injected into and solidified in a mold causing the solid reinforcing elements to be frozen in place. However, if the reinforcing elements are non-uniformly distributed in the liquid composite matrix prior to solidification in the mold, the final distribution of these elements in the solid composite is also non-uniform. Consequently, an inferior composite having non-uniform reinforcing element distribution results from this process, as shown in the prior-art composite of FIG. 1.
Thus, what is needed then are methods of making reinforced composites in which the solid reinforcing elements are uniformly distributed in a composite matrix resulting in a composite matrix wherein the concentration of the solid reinforcing elements in each unit of volume, e.g., cubic millimeter, of the solidified composite matrix is constant throughout the entire composite.
In view of the prior art as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the needed methods of reinforced composite formation could be provided.